NEW YORK (CNN) -- No asbestos was detected in the air Thursday after a steam pipe burst in Midtown in Manhattan the day before, killing one person and wounding 26 others as the ruptured pipe sent thick plumes of smoke and ash into the air.

The city's Office of Emergency Management said, however, that asbestos was detected in six of 10 samples of dust and debris taken from the area.

The city's Department of Environmental Protection will continue to test the debris, according to a statement on the Office of Emergency Management's Web site.

"People who may (have come) into contact with the steam or debris should take a shower and place their clothes in plastic bags for cleaning or disposal," the statement said. "People inside buildings in the frozen zone should keep windows closed and switch air conditioners to recirculate the air inside instead of drawing in air from outside."

The area around the site was "frozen" because of the fear of asbestos in the air.

Those in the frozen zone are allowed to stay in the area, but should use caution. No one will be allowed to enter the zone from outside, the office said.

The New York Fire Department said three firefighters and one police officer were treated on scene for minor injuries. The other 22 injured were transported to various hospitals, a fire department spokesman said.

New York City Mayor Michael Bloomberg said the person who died suffered from cardiac arrest.

The New York Fire Department said it received a call reporting an explosion at 5:56 p.m. More than 170 firefighters were dispatched to the site at Lexington Avenue near 41st Street.

Hundreds of people fled as dozens of police, fire and utility workers arrived. Officials said the explosion was not related to terrorism.

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Carol Bergendale, who witnessed the blast, told WABC that people immediately feared a terror attack. She said there was a loud noise that lasted more than 10 minutes, and people started driving in the wrong direction to get away.

"It felt like an earthquake. We saw hundreds and hundreds of people running down Third Avenue," she said. "They were screaming; they were crying."

Udoji said the blast left a crater "many dozens of feet wide," comparing it to the craters she saw after bombings during a stint in Iraq. She also said a "hissing noise" could be heard for blocks. Blast leaves massive crater »

The emergency response was "instantaneous," she said -- within about 20 minutes the entire area was locked down.

Video from the scene showed steam and mud spewing from underground and a small school bus with its windows and a door blown out.

New York Police Department Deputy Commissioner Paul Browne, speaking to CNN, said the explosion was "major."

"The ground was literally shaking under your feet," he said. Power company ConEdison was checking the area for any asbestos and expected to have test results in Wednesday night.

Andrew Troisi of the New York Office of Emergency Management said people in the area who do not need to be there should leave.

"This is a rescue operation. We have not begun to really detail the possible contaminants in the area, but there's a very good chance that hazardous materials are in the air," he said. "Let's not take any chances."

The pipe that exploded was installed in 1924 and there is speculation that "there was cold water getting into the pipe, and cold water apparently causes these to explode," Bloomberg said.

"It might have been bursts of cold water from the rain or because of another water main break," he said.

ConEdison crews inspected the pipes earlier in the day for vapor conditions and found nothing wrong, company Chairman Kevin Burke said.

ConEdison provides steam power to about 1,800 commercial and residential customers in Manhattan from the Battery to 96th Street, according to the utility's Web site. Steam sales account for less than 10 percent of ConEdison's sales.E-mail to a friend